1. Field of the Invention
This invention relates to devices for detecting the speed of motor vehicles and in the preferred embodiment relates to a system for detecting the speed of motor vehicles for use in traffic control, law enforcement and traffic flow studies.
2. Description of the Prior Art
Speed detection of motor vehicles in the past has been accomplished through the use of expensive radar type devices, the use of computers which calculate speed from time of travel between points of detection, or by the use of pace cars. In the case of radar type devices and computers for determining the speed of motor vehicles, cost is a major limiting factor in their use. In the case of pace cars, the method is unsuited for traffic control or for the continuous monitoring of the speed of motor vehicles. The accuracy is limited by the physical installation of vehicle sensing units and by the accuracy of registering and transmitting the start and stop times to the computer.
In the past, traffic responsive traffic control systems have depended on vehicle count per unit of time for the adjustment in system operation, or they have depended on vehicle presence detection to indicate when vehicles are waiting at a traffic signal. In some cases a combination of the two methods is used.
It is easily shown that count of cars per unit of time has little to do with the speed of traffic flow, i.e., vehicles traveling 40 MPH with a spacing of 108 feet would have the same vehicle count per minute as vehicles with a spacing of 54 feet traveling 20 MPH, or those traveling 60 MPH with a spacing of 176 feet. All would have two second headways (one vehicle each two seconds).
In traffic surveillance and roadway modeling, an attempt has been made to determine the density of traffic flow (lane occupancy) by using a computer to calculate the detector "ON" time (the time it takes a vehicle to pass over the detector) versus the detector "OFF" time (the time in which there is no vehicle over the detector). These calculations have been rough approximations at best, since the sizes of vehicles vary considerably from the largest trucks and trailers to the smallest automobiles and motorcycles. Also, the reaction time (speed of response turning on or off) varies from detector to detector, injecting additional unpredictable errors in the calculations.
Accurately determining the speed (rate of flow) along with the count per unit of time (average headway), and using these in traffic surveillance and traffic control, would have decided advantages over methods now in use. Also, speed detection could be used to apprehend violators of speed laws, provided the accuracy of speed detection were such that evidence would be admissible in the courts. Monitoring of remote speed detection stations from a central office would be possible, using the speed detection system of this invention combined with a system of transmitting the speeds to the central location.